MVC in Rails

# Routes — the entry into the controller
# config/routes.rb
Rails.application.routes.draw do
  resources :orders, only: %i[index show new create] do
    member { post :pay, :ship }
  end
end

# Model — owns persistence, validation, and business rules
# app/models/order.rb
class Order < ApplicationRecord
  belongs_to :customer
  has_many   :items, dependent: :destroy

  enum status: { new: "new", paid: "paid", shipped: "shipped", cancelled: "cancelled" }

  validates :total_cents, numericality: { greater_than_or_equal_to: 0 }
  validates :customer, presence: true

  scope :recent, ->(d = 30.days.ago) { where("created_at > ?", d) }
  scope :open,   -> { where(status: %w[new paid]) }

  def pay!
    update!(status: "paid", paid_at: Time.current)
    OrderMailer.with(order: self).paid.deliver_later
  end

  def ship!(tracking)
    update!(status: "shipped", tracking: tracking, shipped_at: Time.current)
    OrderMailer.with(order: self).shipped.deliver_later
  end

  def total_aud = (total_cents / 100.0).round(2)
end

# Controller — orchestrates a request: load, mutate, render
# app/controllers/orders_controller.rb
class OrdersController < ApplicationController
  before_action :authenticate_user!
  before_action :set_order, only: %i[show pay ship]

  def index
    @orders = current_user.orders.recent.includes(:items).page(params[:page])
  end

  def show; end

  def new;  @order = current_user.orders.build end

  def create
    @order = current_user.orders.build(order_params)
    if @order.save
      respond_to do |fmt|
        fmt.html { redirect_to @order, notice: "Order placed" }
        fmt.json { render json: @order, status: :created }
        fmt.turbo_stream    # renders create.turbo_stream.erb
      end
    else
      render :new, status: :unprocessable_entity
    end
  end

  def pay
    @order.pay!
    redirect_to @order, notice: "Paid"
  end

  def ship
    @order.ship!(params.require(:tracking))
    redirect_to @order, notice: "Shipped"
  end

  private

  def set_order;     @order = current_user.orders.find(params[:id]) end
  def order_params;  params.require(:order).permit(:customer_id, :total_cents, :notes) end
end

# View — formatting, layout, no business logic
# app/views/orders/index.html.erb
<h1>My Orders</h1>
<ul>
  <% @orders.each do |o| %>
    <li>
      <%= link_to "##{o.id}", o %> — $<%= number_with_precision(o.total_aud, precision: 2) %>
      — <%= o.status %>
      <% if o.new? %>
        <%= button_to "Pay",  pay_order_path(o),  method: :post %>
      <% elsif o.paid? %>
        <%= form_with(url: ship_order_path(o)) do |f| %>
          <%= f.text_field :tracking, placeholder: "Tracking #" %>
          <%= f.submit "Ship" %>
        <% end %>
      <% end %>
    </li>
  <% end %>
</ul>

# When the controller grows — extract a Service Object
# app/services/place_order.rb
class PlaceOrder
  def self.call(customer:, items:)
    Order.transaction do
      o = customer.orders.create!(total_cents: items.sum { |i| i[:price_cents] * i[:qty] })
      items.each { |i| o.items.create!(i) }
      Inventory.reserve(items)
      o
    end
  end
end

# Then the controller is one line:
# def create; @order = PlaceOrder.call(customer: current_user, items: params[:items]); end

# Models — db tables / business rules (app/models)
# Views  — ERB templates (app/views)
# Controllers — accept request, call model, render view (app/controllers)